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Abstract
Polyvinyl alcohol (PVOH) exhibits outstanding gas-barrier properties, which favor its use as a biodegradable, high-barrier coating on food-packaging films, possibly in combination with modified atmospheres. Nonetheless, its high sensitivity to water can result in a severe loss of barrier properties, significantly limiting its applications with fresh foods and in high-humidity conditions. In this work, the water vapor (PWV) and oxygen permeability (PO2) of high-barrier biodegradable films with PVOH/PLA + wax double coatings were extensively characterized in a wide range of relative humidity (from 30 to 90%), aimed at understanding the extent of the interaction of water with the wax and the polymer matrices and the impact of this on the permeation process. What is more, a mathematical model was applied to the PWV data set in order to assess its potential to predict the permeability of the multilayer films by varying storage/working relative humidity (RH) conditions. The carbon dioxide permeability (PCO2) of the films was further evaluated, and the corresponding permselectivity values were calculated. The study was finally augmented through modified atmosphere packaging (MAP) tests, which were carried out on double-coated films loaded with 0 and 5% wax, and UV-Vis analyses. The results pointed out the efficacy of the PLA + wax coating layer in hampering the permeation of water molecules, thus reducing PVOH swelling, as well as the UV-shielding ability of the multilayer structures. Moreover, the MAP tests underlined the suitability of the double-coated films for being used as a sustainable alternative for the preservation of foods under modified atmospheres.
Highlights
Plastic pollution is an issue of major concern in our society today
These trends can be interpreted by taking into account the fact that the increase in water vapor partial pressure leads to two opposite effects on the transport phenomena through the film: (i) a compaction effect on the polymer chains due to hydrostatic pressure, resulting in an increase in polymer density, which inhibits the diffusion process; and (ii) a plasticization effect as a direct consequence of the increased concentration, which enhances the segmental motions within the polymer and promotes the diffusion [41–43]
The Biofilm showed the highest PWV values in the whole relative humidity (RH) interval investigated, ranging from 64.0 g mm/m2 d bar at 30% RH to 124.0 g mm/m2 d bar at 90% RH. These values categorize the substrate as a poor barrier-grade material, according to the classification proposed by Wang et al [44], and are in line with the literature data reported for films based on poly(butyleneadipate co-terephthalate) (PBAT)/polylactic acid (PLA) blends [45,46]
Summary
Plastic pollution is an issue of major concern in our society today. According to the2022 OECD Global Plastic Outlook [1], about 22% of all plastic waste worldwide ends up in landfills or as litter, causing significant harm to land and marine environments. Biodegradable polymers are a viable alternative to conventional plastics from a sustainability and circular economy perspective [2]; their performance needs to be enhanced to meet the needs of the market, especially in those sectors where plastic consumption is very high [3]. Polyvinyl alcohol (PVOH) has gained considerable attention among bioplastics due to its unique qualities, including its rapid biodegradability in suitable environments [4–7], good tensile strength, biocompatibility, and outstanding gas-barrier properties [8–11]. These attributes make it highly valuable especially in the food-packaging sector, where providing an adequate gas barrier through active and/or passive technologies is of utmost importance in order to control gas exchange, reduce food oxidation phenomena, and stabilize headspace composition in MAP systems [12–16]. The coating technology, in particular, is garnering a wide interest within scientific and industrial research because it allows one to functionalize biodegradable substrate films in a simple and effective way, conferring them an advanced performance without modifying their bulk properties nor their biodegradable and/or recyclability features [27,28]
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